1. Field of the Invention
The present invention relates to an optical recording disc such as a compact disc and an optical video disc and the like.
2. Description of the Prior Art
A disc recording digital audio data optically, which is called a compact disc, has been known and described in for example "Hoso Gijutsu", April, 1981, pp. 316 and 317 and "Nikkei Electronics", Aug, 17, 1981, pp. 187 and 206.
FIG. 1 shows a construction of the conventional general compact disc. Data are recorded on one surface of a disc-like substrate (1) formed of transparent polycarbonate in the form of pits (dented portions), an aluminium reflecting film (2) being provided on said disk-like substrate (1), and an ultra-violet ray setting type resin protection layer (3) being provided on said aluminium reflecting film (2). And, a label (4) is silk-printed on said resin protection layer (3). The disc is nearly 1.2 mm thick.
And, the data recorded on the above described disc are read out and regenerated as follows. That is to say, in an optical system shown in FIG. 2 a laser beam, which has been irradiated from a semi-conductor laser (5), is turned into parallel lights by a first lens (6) and the resulting parallel lights make a rectilinear propagation through a beam-splitter (7) and then converted from a linear polarized light to a circular polarized light by a 1/4-wave length plate (8) to be collected on a disc (10) through a second lens (9).
The laser beam, which has reached the above described disc (10), is transmitted through the substrate (1) and then reflected by the reflecting film (2) to be again transmitted through the substrate (1) followed by turning into the parallel lights by the second lens (9). Subsequently, the resulting parallel lights are converted from the circular polarized light to the linear polarized light by the 1/4-wave length plate (8) and then arrive at the beam-splitter (7). Such the reflected laser beam that has arrived at the beam-splitter (7) is turned by 90.degree. in progressive direction there to be collected by a third lens (11). The collected reflected laser beam is incident upon a signal-detecting photo-sensor (12), whereby the data are read out to be regene/rated as audio-signals thereof or used as servo signals of a disc-driving motor and pick-up driving means (not shown).
The light incident upon the substrate is generally a red laser beam having a wave length of about 780 nm.
On the other hand, transparent polycarbonate having nearly uniform light transmission characteristics in a visible wave length region, as shown in FIG. 3, has been used as a substrate, not only the red laser beam but also visible rays being easily transmitted therethrough, so that in the case where strong noise lights are incident upon the disc in the visible ray region, they are transmitted through the substrate. If the transmitted noise lights are incident upon the sensor, noise signals are added to the data signals.
As a result, in some extreme cases, there is the possibility that the data are misread and an unstable servo occurs when a sensor output is used as a servo signal.